This invention relates to a semiconductor-on-insulator (SOI) integrated circuit (IC) device, and, more particularly to an SOI IC device having dielectric isolation for the sidewalls of an island, or mesa, disposed on a substrate of an insulator, such as sapphire, for reducing leakage current even in the presence of ionizing radiation, and also to a method for fabrication of the device.
In a typical SOI IC device, an island of a semiconducting material, such as silicon, that is disposed on an electrically insulative substrate, such as sapphire (in which case the device may be referred to as a silicon-on-sapphire (SOS) device) is appropriately implanted with a dopant material imparting the desired type and concentration of majority carriers (e.g. electrons, holes). After a thin, pristine layer of gate oxide is thermally grown over the island, the doped island is ultimately overlaid with a layer of electrically conductive polysilicon for electrically connecting the island with other islands having the same or different semiconductor material and dopants to form an electrical circuit.
In cross section, an island may resemble a rectangle. The polysilicon layer tends to conformally follow the contour of the island so that at the base of the island the electrically insulative substrate, the semi-conducting doped silicon material of the island, an insulative gate oxide layer coating the island and electrically conductive polysilicon are disposed in close proximity to each other. The juncture around the base of the island at the convergence of these materials having diverse electrical characteristics, as well as the silicon/oxide interface, is subject to trapped charges and concentrated electric fields which may result in undesirable leakage currents during circuit operation, especially in the presence of ionizing radiation. Although generally not considered to be as severe, undesirable leakage currents may also arise at the juncture of the top and side wall of the island that is overlaid with polysilicon.
It would be desirable to eliminate sources of potential leakage current without affecting overall operation of the device. In addition, it would be desirable to provide a method for fabricating a device exhibiting improved leakage current characteristics, wherein the method is both economical and compatible with existing technology for manufacture of IC devices.
Accordingly, it is an object of the present invention to reduce the propensity of a SOI IC device to exhibit leakage current from the convergence of the insulative substrate and the side-wall of an island even in the presence of ionizing radiation.
It is another object of the present invention to reduce leakage current from the juncture of the top and side wall of an island of an SOI device.
It is yet another object of the present invention to provide a method for fabricating a silicon-on-insulator integrated circuit having reduced leakage current, wherein the method is economical and may be incorporated into conventional processing systems for SOI devices.